Voltage tuned cold cathode secondary emission delay line oscillator



g- 1967 R. c. HERGENROTHER 3,334,310

VOLTAGE TUNED COLD CATHODE SECONDARY EMISSION DELAY LINE OSCILLATOR 2 Sheets-Sheet 1 Filed Aug. 17, 1965 7 PRIOR ART PRIOR ART II I I jI/I/l/l/II' INVENTOR DOLF CHERGBVMHB? V A7TOR/VEY Aug. 1, 1967 R. c. HERGENROTHER 3,334,310

VOLTAGE TUNED COLD CATHODE SECONDARY EMISSION DELAY LINE OSCILLATOR 2 Sheets-Sheet 2 Filed Aug. 17, 1965 R F OUTPUT INVEA/ rm? RUDOLF c. HERGENROTl-IER BY 2 i ATTORNEY United States Patent 3,334,310 VOLTAGE TUNED COLD CATHODE SECONDARY EMISSION DELAY LINE OSCILLATOR Rudolf C. Hergenrother, West Newton, Mass., assignor to Raytheon Company, Lexington, Mass., a corporation of Delaware Filed Aug. 17, 1965, Ser. No. 480,265 11 Claims. (Cl. 331-82) ABSTRACT OF THE DISCLOSURE I The invention relates generally to self-oscillating electron discharge devices of the cold cathode secondary emitter type and has particular reference to the incorporation of a slow wave periodic structure within such devices to thereby result in a high frequency and broadband oscillation generator which is voltage tunable.

Electronic oscillator tubes capable of converting direct current energy into oscillations at high frequencies have been suggested in the art which operate without the use of electron gun emitters or heated cathodes. Such electron discharge devices are provided with a plurality of dynode or secondary emitter electrodes within a highly evacuated envelope. The electrode surfaces are sensitized to have a low work function and are especially suited for the liberation of large numbers of secondary electrons at a ratio greater than unity when bombarded by traveling electrons. Commonly, the electrodes are oppositely disposed with an anode disposed therebetween which is wholly or partially permeable to the electrons passing in the region between the electrodes. Interconnection of such electrodes to appropriate potentials and an external resonant LC tuned circuit or a Lecher system will result in a cloud of electrons oscillating back and forth in a potential valley in accordance with the well known Barkhausen retarding field principles to produce periodic electron currents and high frequency oscillations. The period of such oscillations is determined by the transit time of the electrons between the electrodes which is dependent upon the voltages and period of the external resonant circuits connected thereto. The movement of the electrons within the tube can be conceived as a pendulum movement in that the population of electrons in the tube will be continually regenerated until a final state is reached determined bythe electrode yield and the space charge in the interelectrode region. For this reason such devices are sometimes referred to as a pendulator and an exemplary device is disclosed in the United States Patent No. 2,184,910, issued to P. T. Farnsworth, on Dec. 26, 1939.

The so-called Farnsworth pendulator type tube is somewhat limited in its frequency capabilities due to the generation of heat during operation causing irreversible changes in secondary emission as well as the excessive noise created by the large spread of the emission velocities of the secondary electrons within the envelope. Further, such devices are dependent on external resonant circuitry for the determination of the frequency of the oscillations and such circuit tuning has limitations.

The present invention discloses a new concept in the provision of a self-contained voltage tunable oscillation 3,334,310 Patented Aug. 1, 1967 generator without cathodes or electron gun emitters relying solely on secondary electron emission.

A further object of the present invention is the provision of an electron discharge oscillator device incorporating an internal periodic slow wave interaction circuit disposed between the secondary emitter electrodes utilizing the principles of high frequency wave generation in such interaction circuits.

Still another object of the present invention is the provision of an electron discharge device having secondary emitter electrodes and means defining a sinuous energy exchanging interaction structure disposed therebetween capable of inducing the generation of high frequency electromagnetic wave energy upon interaction of the emitted secondary electrons with the electric fields established in the interaction structure.

In accordance with the teachings of the present invention a periodic slow wave structure of appreciable axial length is disposed within an evacuated envelope between cold cathode electrodes having a secondary emission ratio greater than unity. By means of external magnetic fields or electrostatic focusing the emitted secondary electrons are guided along the path defined by the periodic. structure to interact with the electric fields established thereon and thereby provide an internal feedback mechanism resulting in self-sustained oscillations at a frequency substantially determined by the velocity of the electrons in relation to the selected periodic structure. Within the last decade the interaction phenomena of a beam of electrons with the electric fields of periodic structures as well as improvements in design of such structures has evolved in the generation of high power energy oscillations as well as amplification. Accordingly, the concept has come to be known that when a high frequency wave is induced or propagated along an interdigital delay line periodic structure, interaction occurs between the electrons and the electric fields having a delay or phase velocity substantially equal to the velocity of the electrons in a beam. The flow of such an electron beam also induces in the periodic structure a space harmonic which is directed in the opposite direction to the electron beam. Thi space harmonic interacts with the beam to produce further oscillations or amplifications and the energy is caused to flow in a backward direction. The increased flow of energy further enhances additional oscillations and if the population of electrons is sufiiciently high this internal feedback mechanism results in self-sustained oscillations at very high frequencies. Utilization of the backward wave mode of oscillation results in the frequency of oscillations being varied simply by varying the velocity of the electrons which is in turn controlled by the voltage potentials between the electron emitters. This results in a voltage tunable secondary emission electron discharge device which eliminates the expense and limitations of external resonant circuits employed in prior art devices and at the same time provides a superior product.

An illustrative periodic slow wave structure includes an interdigital delay line; however, numerous other types of such periodic structures, such as bifilar helices and ladder lines, may be employed in the practice of the invention. The use of electrostatic as well as magnetic fields for the focusing of the electron cloud in close proximity to the periodic structure will also be featured.

Other objects, features and advantages will be evident after consideration of the following detailed description of the invention together with the accompanying drawings in which:

FIG. 1 is a cross-sectional view of an illustrative embodiment of a prior art structure employing secondary emitter electrodes together with an external tuned resonant circuit and magnetic focusing means;

FIG. 2 is a cross-sectional view of an illustrative embodiment of a prior art structure employing electrostatic focusing means;

FIG. 3 is a schematic presentation of an illustrative embodiment of the present invention employing magnetic focusing means;

FIG. 4 is an enlarged longitudinal cross-sectional view of an illustrative periodic delay line structure employed in an embodiment of the invention;

FIG. 5 is a detailed cross-sectional view along the line 5-5 in FIG. 4; and

FIG. 6 is a longitudinal cross-sectional View of an illustrative embodiment of the invention employing electrostatic focusing.

An exemplary embodiment of a secondary emission col-d cathode device is shown in FIG. 1. Envelope 2 is highly evacuated and houses oppositely disposed cold cathode secondary emitter electrodes 4 and 6 in the shape of discs. The opposing surfaces of electrodes 4 and 6 are sensitized so as to emit secondary electrons in great quantities upon electron impact. Commonly, a secondary yield per incident primary electron collision exceeds unity and as many as ten secondaries may be liberated when silver electrodes are oxidized with a layer of cesium deposited thereon. Other satisfactory secondary emissive materials include aluminum or platinum, as well as specially treated alloy metals bearing alkaline earth metal oxides. An anode 8 interposed between electrodes 4 and 6 defines a substantial portion permeable to the electrons passing in the intermediate region.

Magnetic field producing means 10, such as a magnet or solenoid, may be provided with voltage source means 11 and an adjustable current limiting resistor 13. A field is established extending longitudinally to the envelope axis to prevent interruption of the electron flow by the anode and as a result the electron flow is confined along the axial path. Arrow B indicates the desired direction of the magnetic field.

Suitable external circuitry for the operation of the tube includes a voltage source 12 positively biasing anode 8 through a choke coil 18 to present a high impedance to the oscillations in an external resonant circuit. Electrodes 4 and 6 are negatively biased and interconnected to an induction coil 14 in shunt with an adjustable con-denser 16 to facilitate the tuning of the inductance and capacitance of this LC resonant circuit. An output pickup coil 19 is coupled to the LC circuit coil 14 to provide means for connection of the generated high frequency oscillations to a utilization load.

The potential differences or the field intensities established between the electrodes provide the kinetic energy to release the secondary electrons which may be simply considered as a group of pebbles moving back and forth in a positive trough. When one of the electrode-s is initially biased a few electrons (photo electrons) may first be emitted. This group of electrons is accelerated by means of the adjacent anode electrode and a group of these electrons arrive at the opposing electrode to bombard the sensitized surface and release secondary electrons. These released and initial electrons return again to the first electrode and produce a still greater population. The process is repeated many times until a final state is reached determined by the yield capability of the electrodes or the space charges in the interelectrode region.

The external resonant LC circuit coupled between the electrodes results in the establishment of a negative conductance at the terminals thereof and self-oscillation at the resonant frequency will be established as the circuit either delivers or absorbs energy from the electron discharge tube. As a result of the repetition of the secondary electron emission process, an electron cloud is created which will tend to move to and fro as the potentials on respective electrodes are altered and thereby establish oscillations in the tuned circuit. The period of oscillation is dependent on the transit time of the electrons in the interelectrode region which in turn is dependent upon the resonances of the external tuned circuit.

FIG. 2 illustrates another embodiment of the prior art device-s in which the electrodes 22 and 24 are shaped concavo-convex with the anode 26 in the form of a centrally disposed spherical electrode consisting of a plurality of fine wires to form a mesh grid arrangement. The envelope 20, as in the previous embodiment, is highly evacuated. Electrodes 22 and 24 provide a focusing action and an electrostatic field to assist in the directing of electrons from one electrode to the opposing electrode in passing through the Faraday space formed by the anode 26. The external tuned circuit indicated generally by the numeral 28 is similar to the circuit previously discussed in connection with FIG. 1. In this embodiment the external magnetic field is eliminated and the confinement of the electrons in the central portion of the tube envelope is provided by means of electrostatic focusing.

Turning next to FIGS. 3, 4 and 5, the illustrative embodiment of the invention is disclosed to obtain higher frequency oscillations over broader bandwidths than was heretofore attained due to the limitations of external resonant circuits. Envelope 30 which may be metallic as well as dielectric is provided with axially disposed secondary emitter electrodes 32 and 34 fabricated of the same material and sensitized as the prior art electrodes. In this embodiment with a metallic envelope conductors 36 and 38 are insulated from the envelope and connect the electrodes to a variable source of direct current 54. Choke coils 57 and 58 are provided to present a high impedance to the high frequency oscillations generated within the tube. Spaced within the interelectrode region is a nonresonant periodic slow wave structure of appreciable length which has been shown for illustrative purposes as an interdigital delay line comprising a first and second plurality of interlocking finger members 40 and 42. Such delay lines closely resemble oppositely disposed combs with the finger members of each group electrically coupled together to define a sinuous energy exchanging interaction path along the longitudinal axis of the envelope.

In FIG. 4 a specific structure is shown comprising a plurality of finger members 40 coupled together and opposing alternately interleaved finger members 42 adjacent thereto. To assist in the directing of the secondary electrons along the delay line path a central portion 55 of each finger member may be widened to provide for an aperture 52 through which the beam can pass. The flow of the beam of secondary electrons is indicated by the dotted lines 50. In accordance with the practice of the invention the interdigital delay line finger members may be mounted directly on the tube envelope which may be at ground potential to provide the accelerating fields necessary for the traversal of the electrons along the beam path within the tube.

Magnetic field producing means 48 surrounds the envelope and may be a solenoid or permanent magnet arrangement well known in the art.

While a specific slow wave structure has been illustrated, the invention is not limited to this configuration but is intended to embrace all other such structures such as 'bifilar helices or ladder-type delay lines employed in backward wave oscillators and traveling wave tubes of the O-type employed in microwave oscillators and amplifiers with heated cathodes and gun emitters. Modifications may be necessary where a dielectric envelope is employed and the metallic delay line structures are suitably supported therein. Further modifications may also be practiced in the method of coupling the delay line members to the appropriate voltages desired.

In operation the initial voltage potentials are established between the electrodes 32 and 34 and the delay line. Primary electrons produce secondaries to result in the electron cloud or beam 50 defined along the axial path between said electrodes. It is important in the case.

of secondary emission type devices that the delay line finger adjacent to the electrodes at each end be at a positive potential to thereby accelerate the escape of the secondaries from the electrode surfaces. Voltage source 54 connected to choke coil 56 will provide the necessary biasing. As the electrons traverse the delay line path the waves established thereon come into play until the velocity of the electron movement is substantially equal to the apparent or phase velocity of the delay line waves. In the backward Wave mode interaction will result between the beam and a negative space harmonic for a sustained flowof energy in a direction opposite to the flow of electrons. Continual interaction results in oscillations being established by the internal feedback system thus established on the delay line to finally result in selfsustaincd alternating current oscillations at a frequency wholly determined by the velocities of the electron beam in relation to a given delay line structure. Such oscillations are in the backward wave mode and are easily adjusted by variable tuning of the voltage potentials between the electrodes. The high frequency oscillations generated may be coupled to a utilization load by means of the connection of a coaxial transmission line having an outer; conductor 44 coupled to the envelope 30 and an inner conductor 46 coupled to the interdigital delay line finger adjacent to the electrode 34.

Referring next to FIG. 6, an alternative embodiment of the invention is shown which eliminates the need for external magnetic focusing means and relies solely on electrostatic focusing. In addition, this embodiment discloses a more symmetrical arrangement with the voltage inputs and output coupling means centrally disposed. The cold cathode secondary emitter electrodes or dynodes 62 and 64 may be supported at the opposing ends of interdigital delay line segment member 70 defining a plurality of finger members 72. Another similar structure electrically isolated therefrom comprises a segment member having a plurality of finger members 68 interleaved between the fingers of the first-mentioned member. Apertures 74 are again indicated in all of the finger members. Member 70 may be negatively biased by a variable source of direct current 76 and choke coil 78, while member 66 is positively biased by means of connection 86 attached to the envelope which is biased at a positive potential. In this arrangement adjacent delay line fingers are at alternate potentials to define a series of electron lenses and by suitable adjustment of the voltage differences between the adjacent sets of delay line finger members the strength of the converging lens systems can be adjusted to balance the beam-spreading forces of the space charge. Parallel fiow of the electrons can therefore be maintained along the length of the delay line structure. This voltage adjustment may be simply made through control of the voltage source 76. The output coupling means for the high frequency energy generated is indicated by connections to a central finger member of the delay line and the inner conduct-or 84 of the coaxial transmission line having the outer conductor 88 coupled to the envelope. The voltage I supply means to the internal periodic slow wave structure members may also be coupled to the tube envelope through a connector 82 with the electrical conductor 80 insulatedly supported therein.

While a specific embodiment of the invention has been described, many modifications and alterations may be evident to skilled artisans. It is, therefore, intended that the scope of the invention be interpreted in accordance with the broadest aspects of'the definition thereof in the appended claims.

What is claimed is:

1. In combination:

an evacuated envelope;

oppositely disposed cold cathode sources adapted to emit secondary electrons at a rate greater than unity upon electron bombardment disposed within said envelope;

a periodic slow wave structure disposed adjacent to said source to define an energy exchanging interaction path along the longitudinal axis of said envelope;

means for establishing voltage potential differences between said sources and said slow Wave structure to initiate and sustain secondary electron emission;

means for producing an electric field adjacent to said path to induce generation of high frequency alternating current oscillations upon interaction of the secondary electrons with said field;

and means for coupling said oscillations from said slow wave structure.

2. An electron discharge device comprising:

an evacuated envelope;

a plurality of spaced oppositely disposed cold cathode electrodes adapted to emit secondary electrons at a rate greater than unity upon electron bombardment enclosed within said envelope;

a periodic slow wave structure defining a sinuous energy exchanging interaction path in the region between said electrodes;

means for establishing voltage potential differences be tween said electrodes and said slow wave structure to initiate and sustain secondary electron emission;

means for providing an electric field adjacent to said path to induce generation of high frequency alternating current oscillations upon interaction of the secondary electrons with said field;

and means for coupling said oscillations from said slow wave structure.

3. An electron discharge device comprising:

an evacuated envelope;

a plurality of spaced oppositely disposed cold cathode secondary electron emitter electrodes enclosed within said envelope;

a periodic slow wave structure defining an extended sinuous energy exchanging interaction path in the region between said electrodes;

means for establishing voltage potential differences between said electrodes and said slow wave structure to initiate and sustain secondary electron emission;

means for focusing the secondary electrons along said path;

means for biasing said slow wave structure at a direct current potential to produce an electric field along said path to induce generation of high frequency alternating current oscillations upon interaction of the secondary electrons with said field;

and means for coupling said oscillations from said slow wave structure.

4. An electron discharge device comprising:

an evacuated envelope;

a plurality of spaced oppositely disposed cold cathode electrodes having sensitized surfaces adapted to continually emit secondary electrons upon electron bombardment enclosed within said envelope;

a periodic slow wave structure defining an extended sinuous energy exchanging interaction path between said electrodes along the longitudinal axis of said envelope;

means for establishing a magnetic field parallel to the longitudinal axis of said envelope to focus the secondary electrons along said path;

means for biasing at least the portions of said slow wave structure adjacent to saidelectrodes at a positive direct current potential to assist in the emission of secondary electrons and to produce an electric field along said path to induce generation of high frequency alternating current oscillations upon interaction of the secondary electrons with said field;

and means for coupling said oscillations from said slow wave structure.

5. An electron discharge device according to claim 4 potentials to vary the period of said oscillations.

6. An electron discharge device comprising: an evacuated envelope; a plurality of spaced oppositely disposed cold cathode ing current oscillations upon interaction of said secondary electrons with said field;

and output coupling means connected to at least one of said members to utilize said oscillations.

9. A traveling wave oscillator tube comprising:

an evacuated envelope;

cold cathode secondary electron emitter electrodes disposed at opposite ends of said envelope;

means for biasing said emitter electrodes negatively to initiate and sustain the emission of secondary electrons;

an interdigital delay line comprising a plurality of interlocking members to define an energy exchanging electrodes having sensitized surfaces adapted to coninteraction path between said electrodes, each of said tinually emit secondary electrons upon electron bommembers defining an aperture aligned with the longibardment enclosed within said envelope; tudinal axis of said envelope; said cold cathode electrodes being negatively biased to m gn i fi l producing e n x rn l t sai eninitiate and sustain secondary electron emission; velope to direct the secondary electrons alon Said a periodic slow wave structure having a first plurality 1O P of members electrically coupled together and a secvoltage source means biasing said delay line members 0nd plurality of members electrically coupled topositively with respect to said electrodes to progether, said fir t and second pluralities b i i an duce an electric field adjacent to said path and induce interlocking array to define an extended sinuous engeneration of high frequency Oscillations in the backergy exchanging interaction path between said elec- Ward wave mode upon interaction of said secondary trodes; electrons with said field; means for biasing adjacent slow wave structure mema o tp t coupling means connected to at least one bers at alternate positive and negative direct current of Said delay line members t0 UtiliZe Said Oscillations. potentials to establish an electrostatic focusing field A traveling Wave Oscillator tube according to Claim to direct secondary electrons along said path and 9 and control means to vary the voltage potentials beinduce generation of high frequency alternating curtween said delay line members and said emitter elecrent oscillations upon interaction of the secondary trod selectrons with said electrostatic field; 11. A traveling wave oscillator tube comprising: and means for coupling said oscillations from said an evacuated finVelOpe;

slow wave structure. cold cathode secondary electron emitter electrodes dis- 7. An electron discharge device according to claim 6 pose a opposite ends of said envelope; and means for adjusting the current potential values besaid emitter electrodes being negatively biased; tween adjacent members of the slow wave structure to an interdigital delay line comprising a plurality of invary the period of aid o illation terlocking members to define an energy exchanging 8. A traveling wave oscillato tube comprising; interaction path between said electrodes, each of said an evacuated envelope; members defining an aperture aligned with the longicold cathode secondary electron emitter electrodes distudinal axis of said envelope;

posed at opposite ends of said envelope; voltage source means biasing adjacent delay line meman interdigital delay line comprising a plurality of inbars at alternate Positive and negative biasing P terlocking members to define an energy exchanging tentials to produce an electrostatic focusing field adinteraction path between said electrode jacent to said path and induce generation of high fremeans for establishing voltage potential differences beq y Oscillations ill the backward Wave mode 1113011 tween said emitter electrodes and said delay line to interaction of said secondary electrons with said field; initiate and sustain the emission of secondary elec- 40 control means for varying the voltage biasing potentrons; tials between adjacent delay line members; means for directing the secondary electrons along said and output coupling means connected to at least one path; of said positively biased delay line members to utilmeans for producing an electric field adjacent to said iZe said oscillation.

path to induce generation of high frequency alternat- References Cited UNITED STATES PATENTS 2,881,348 4/1959 Palluel 3l53.6,

ROY LAKE, Primary Examiner.

S. H. GRIMM, Assistant Examiner. 

1. IN COMBINATION: AN EVACUATED ENVELOPE; OPPOSITELY DISPOSED COLD CATHODE SOURCE ADAPTED TO EMIT SECONDARY ELECTRONS AT A RATE GREATER THAN UNITY UPON ELECTRON BOMBARDMENT DISPOSED WITHIN SAID ENVELOPED; A PERIODIC SLOW WAVE STRUCTURE DISPOSED ADJACENT TO SAID SOURCE TO DEFINE AND ENERGY EXCHANGING INTERACTION PATH ALONG THE LONGITUDINAL AXIS OF SAID ENVELOPE; MEANS FOR ESTABLISHING VOLTAGE POTENTIAL DIFFERENCES BETWEEN SAID SOURCES AND SAID SLOW WAVE STRUCTURE TO INITIATE AND SUSTAIN SECONDARY ELECTRON EMISSION; MEANS FOR PRODUCING AN ELECTRIC FIELD ADJACENT TO SAID PATH TO INDUCE GENERATION OF HIGH FREQUENCY ALTERNATING CURRENT OSCILLATIONS UPON INTERACTION OF THE SECONDARY ELECTRONS WITH SAID FIELD; AND MEANS FOR COUPLING SAID OSCILLATIONS FROM SAID SLOW WAVE STRUCTURE. 